Evidence indicates that certain animals detect and orient themselves to the earth's magnetic field. This capacity may be used in the context of homing, finding mates, and locating food sources, for instance. Although the existence of biosensory apparatus for transducing geomagnetism can be inferred from the orienting responses of diverse animals, little is yet known of the mechanisms of transduction of geomagnetic energy into the nervous system of any multicellular animal. An animal has been found which has two unusual features and which provides an opportunity to achieve an understanding of the physical/biological basis for the geomagnetic sense. The marine nudibranch mollusc, Tritonia diomedia responds to the earth's magnetic field by orienting appropriately when other cues are unavailable. Secondly, in recent experiments, a symmetrical pair of re-identifiable neurons in the animal's brain have been found which respond to changes in the direction of the ambient magnetic field by firing electrical impulses. In this proposal, experiments are described at the behavioral, neurophysiological and molecular level that are designed to (1.) establish the normal behavioral context and use of geomagnetic information, (2) establish interaction between sensory inputs carrying information about rheotactic (water current) and geomagnetic cues, and (3) cellular/molecular experiments to determine the physiological roles of unique peptides found in neurons involved in magnetosensory integration. Little is know of the sensory or other processes in animals which mediate interactions with the earth's magnetic field, or with stronger magnetic fields. These studies may help elucidate these relationships.